1. Field of the Invention
This invention relates to a screw type vacuum pump with a check valve in a fluid passage on the suction side thereof.
2. Prior Art
Screw type vacuum pumps are generally constituted by a pair of intermeshed male and female screw rotors, a pump casing having a suction port and a discharge port on the opposite sides thereof, an overdrive mechanism for increasing the speed of the rotational driving force of a motor before transmission to a rotor, an overdrive casing serving also as an oil tank, and an oil circulating passage passing through a number of lubricant oil supply points in the pump casing, overdrive gears, oil tank, oil pump and oil cooler. A screw type vacuum pump of this sort is known, for example, from U.S. Pat. No. 4,767,284.
More specifically, as illustrated in FIG. 3, the existing screw type vacuum pump of this sort has a pair of intermeshed male and female screw rotors 19 rotatably accommodated in a casing 18 which is provided with a suction port 13 on one side and a discharge port 17 on the other side, the rotors being rotationally driven in one direction, for example, by a rotor shaft 20 which is extended through and out of the casing 18 on the side of the suction port 13. As indicated by an arrow of solid line in the same figure, a gas is taken into the pump casing 15 through the suction port 13 and discharged through the discharge port 17. However, should a suction force acting in a direction inverse to the solid line arrow occur at the suction port 13 as indicated by an arrow of broken line, gas would flow in through the discharge port in the direction of the broken line arrow, rotating the rotors in reverse directions. This happens when a vacuum tank 26 is directly connected to the suction port 13 of the vacuum pump 12 as shown by imaginary line in FIG. 3, namely, the rotors are rotated in reverse directions when the operation of the pump 12 is stopped, due to a pressure difference between the vacuum tank 26 and the atmosphere, permitting air to flow into the tank 26.
The time of reverse rotation on such an occasion is determined depending upon the size of and the pressure in the tank 26. This phenomenon of reverse rotation imposes adverse effects on bearings of the rotors and other associated parts. Namely, since the lubricant oil is supplied to the bearings and synchronous gears by an oil pump which is generally driven in synchronism with the drive source of the vacuum pump, the oil pump is stopped as soon as the vacuum pump comes to a stop.
Therefore, the rotors are rotated in reverse directions without supplying the lubricant oil to the bearings and other parts which need lubrication, imposing adverse effects thereon.
The reverse rotor rotation phenomenon also occurs in the following circumstances.
In a case where a plural number of screw type vacuum pumps 12 have their suction ports connected to a common suction passage 11 through respective branch passages 14 as shown in FIG. 4, if one pump 15 in the rightmost position in the figure alone is stopped, its rotors are caused to rotate in reverse directions by the suction forces of the two other pumps which are in operation, as indicated by an arrow of broken line. The reverse rotation of the stopped pump 15 might lead to a breakdown since it is put in operation without supply of lubricant oil to its bearings, synchronous gears and other parts which need lubrication.
In order to prevent such a situation resulting from reverse rotation of a pump 15, it is the usual practice to install a check valve 16 in each branch passage 14.
In case the check valve 16 is a commercial product, it is generally constituted by, as shown in FIG. 5, a valve casing 23 internally providing a gas flow space 22 with a valve seat 21, a valve body 24 intimately seatable on the valve seat 21, and a coil spring 25 constantly urging the valve body 24 toward the valve seat 21 for intimate engagement therewith. In this case, when a suction force exists at the suction port 13, namely, when a suction force acts in the direction of port x in the drawing, the valve is opened and gas flows from port y to port x. Conversely, when a suction force comes from other pump 15, namely, when a suction force acts in the direction of port y, the valve body 24 is held in intimate contact with the valve seat 21, thereby closing the valve to block reverse gas flows.
If the spring constant of the coil spring 25 is small, however, the vacuum pump 12 with the check valve 16 of the above-described construction suffers from a time delay in closing the valve when the suction force is reversed toward the port y, failing to completely preclude the reverse rotation of the pump 15.
On the contrary, when the spring constant is increased, there arises a problem that the valve body 24 is repeatedly hit against the valve body 21 due to the low density of influent gas in the vacuum device, causing the hunting phenomenon in which the valve is opened and closed repeatedly.